The world's largest class superconducting coil is used for the "Large-scale Helical Device". Its electrical insulation system might be exposed to considerably severe multiple stresses including cryogenic temperature, large mechanical stresses and strong magnetic fields. It is therefore very important to study its electrical insulation performance in order to establish the reliability of the coil. If a superconductor quenches from superconducting state to normal state, the liquid coolant vaporizes very easily and turns into high-density gas at cryogenic temperature, which may reduce its withstanding voltage. Furthermore, it is very difficult to completely remove foreign particles from the insulated space. So it is required to clarify the influence of foreign particles and electrification on the insulation performances.This research was conducted using electrode system that simulated the insulation system to investigate the behavior of small gaps and foreign particles with the breakdown characteristics of insulation. The breakdown voltage of insulation system that has a small gap in liquid helium is shown in Fig.1. The breakdown voltage increased with decrease of voltage rising rate when the polarity of voltage was positive, but the breakdown voltage was almost constant with decrease of voltage rising rate when the polarity of voltage was negative. Assuming that positive charge can move along the spacer surface that is in contact which the metallic electrode, lower rising rate of applied voltage would bring more significant charge accumulation on the spacer, leading to the field enhancement which reduces the breakdown voltage. It became also clear that conductive foreign particles affect the breakdown voltage while dielectric foreign particles do not.